1. Field of Invention
The present invention relates to a method and a composition for the suppression of bloom in peroxide crosslinked polymeric articles and, more particularly, to the suppression of bloom in polymeric articles that are crosslinked using α,α′-bis(tert-butylperoxy) diisopropylbenzene.
2. Description of Related Art
The term “bloom” refers to a thin coating of an ingredient of a rubber or other thermosetting polymeric mixture that migrates to the surface of an article, usually within a few hours after crosslinking. Bloom is usually harmless, but it can impair the aesthetic appearance of crosslinked polymeric articles and can sometimes interfere with the ability to bond or adhere materials to the crosslinked polymeric articles.
One of the ingredients of polymeric compositions that is known to cause bloom is α,α′-bis(tert-butylperoxy) diisopropylbenzene, a mixture of the meta and para isomers of that is widely used as a crosslinking agent for a variety of polymers including, for example, ethylene propylene terpolymer rubber (“EPDM”). Like all organic peroxide crosslinking agents, α,α′-bis(tert-butylperoxy) diisopropylbenzene crosslinks polymers by decomposing at relatively high temperatures to generate free radicals, which form crosslinks between polymer strands via a hydrogen abstraction mechanism. The decomposition products of α,α′-bis(tert-butylperoxy) diisopropy-benzene include the meta and para isomers of diacetylbenzene, acetyl dimethylbenzyl alcohol and diisopropylol benzene, the latter of which is a solid at room temperature (˜22.5° C.) and is believed to be a major constituent of bloom.
Bloom from the decomposition of α,α′-bis(tert-butylperoxy) diisopropylbenzene may not be noticeable, particularly when low concentrations of α,α′-bis(tert-butylperoxy) diisopropylbenzene are used, because the decomposition products do have some solubility in even the most non-polar polymer matrices. But at in many crosslinked polymeric articles, at least some of the decomposition products tend to slowly migrate to the surface of the crosslinked polymeric article and form bloom. Such bloom can have a varied appearance including “iridescent speckling” to “dusting” to “hair-like filaments” that appear to emerge from the surface of the crosslinked polymeric article. The “hair-like filaments” appear because the blooming decomposition products form crystals when they reach the surface of the crosslinked polymeric part. In many cases, these crystals are oriented substantially perpendicular to the surface of the crosslinked polymeric article, which makes their presence particularly conspicuous. Bloom from the decomposition of α,α′-bis(tert-butylperoxy) diisopropylbenzene usually becomes evident on the surface of a crosslinked polymeric article several hours after vulcanization or curing, although in some cases it may take several weeks to become noticeable.
As previously noted, bloom is not desirable, if only for aesthetic reasons. In some cases, however, it may interfere with bonding between two crosslinked polymeric articles since the bloom may form a barrier that inhibits adhesion between the two articles. Bloom is conventionally removed from crosslinked polymeric articles by water washing. This solution, which adds a processing step and creates a nuisance waste stream, is often a temporary solution inasmuch as the bloom reappears over time.